Intercommunication system



March 7, 1950 w. R. TORN INTERCOMMUNICATION SYSTEM 2 Sheets-Sheet 1 Filed Nov. 9, 1946 lllil kt r INVENTOR. WILLIAM FL TORN W. R. TORN INTERCOMMUNICATION SYSTEM March 7, 1950 2 Sheets-Sheet 2 Filed Nov. 9, 1946 SYSTEM CABLE 5 Bus:

INVENTOR. WILLIAM R TORN @m L mm 2 mxaw mmm 5N 5:26 mo5w mw ZOE/Em Patented Mar. 7, 1950 INTERCOMMUNICATION SYSTEM William R. Torn, St. Charles, 111., assignor to Operadio Manufacturing Company, St. Charles, 111., a corporation of Illinois Application November 9, 1946, Serial No. 708,852

5 Claims.

This invention relates to an electrical apparatus and particularly to an inter-communicating system useful for establishing communication among a number of stations.

Inter-communicating systems generally comprise one or more master stations and one or more remote stations, all inter-connected to form a system. It is possible to have all inter-connected stations as master stations, or a number of master stations and remote stations, or one master station and remote stations. As is well known in the art, a master station includes suitable amplifying means and switching means for selecting the called station or stations. A remote station generally consists of a transducer and a switch for changing from talking to receiving.

Such inter-communicating systems have come into substantial use. It has been found, however, that inter-communicating systems of the prior art lack flexibility to permit them to be adapted to various systems desired by customers. In most inter-communicating systems, it is possible for the originating station to connect one or more other stations for communication. Thus, the

load on the originating master station may vary over wide limits. It has been found in systems of the prior art that such wide variation of load results in improper matching and results in wide variations of quality and volume. In such systems, one setting of a volume control at a master master station. Unless such other station is another master station with a volume control, such other remote station can do nothing except communicate this condition to the originating station.

It is frequently desirable to provide means in an inter-communicating system for transforming one or more stations into a high gain station supplied with a power speaker for paging purposes. The systems of the prior art do not provide simple means for accomplishing this.

A system embodying the present invention overcomes the preceding disadvantages and may be readily installed in an expeditious and economical manner. An inter-communicating system embodying the present invention has unusual flexibility permitting a, composite communication system of master and remote stations as desired. Such a system provides means whereby the level of volume will remain substantially independent of the number of stations in communication at any one time. A system embodying the present invention permits a master station to adjust the volume level to a satisfactory value, which value will remain substantially constant under all conditions of use.

A system embodying the present invention has a further advantage of providing abnormally high gain at a station; The abnormally high gain may be used for paging.

A system embodying the present invention provides convenient circuits whereby prewired units may be installed to form a complete system independently of the number of master or remote stations. In systems of the prior art, this has generally required special wiring assemblies from the factory for a particular type of system. In distinction to this, a system embodying the present invention may have any number of master or remote stations to form a complete system up to the limit of the capacity of the system. The same prewired units, by a simple change, may have from one to the maximum number of master stations or mixed master and remote stations.

A system embodying the present invention has additional advantages in that a master station provides for a minimum number of circuit components. In particular, a system embodying the present invention relies upon cathode injection in distinction to the normal grid injection in a vacuum tube amplifier. By virtue of this, matching between a transducer and a vacuum tube is simplified and eliminates a matching transformer. This is particularly important since transformers in intercommunicating systems are susceptible to noise and power line pick-up unless precautions are taken to shield the transformer.

The drawings show a, circuit diagram of a master station, Figure 1 being one portion of the circuit and Figure 2 being the remaining portion of the circuit.

Referring now to the master station, transducer ill has one terminal connected to grounded junction ll. Transducer l 0 may be any reversible loud speaker having sufiicient sensitivity to be used as a microphone. Thus, a dynamic type of loud speaker with a permanent magnet field is generally used for this purpose. However, other types of translating devices may be used and, inasmuch as such devices are well known in the 3 art, no detailed description thereof is deemed to be necessary.

Transducer I- is connected through leads LI and MI to terminal I3 of resistor l5 having an impedance of the order of five or more times that of transducer I0. Resistor I5 has its other terminal I9 connected through wire I! to resistor I8 having an impedance about equal to that of transducer I0 and thence, by wire Hi, to switch contact 2I' of a talk-listen switch assembly generally indicated by numeral 23.

Cooperating with resistor I5 is potentiometer wiper connected through wire 26 to switch contact 21 of switch 23.

Grounded junction II has connected thereto ground wire RI. Between ground wire RI and junction point 30 is connected resistor 3|. Junction 30 is also connected through condenser 32 to wire 33 going to junction 34 and continuing to switch contact 35 of switch 23. Between wire 33 and ground wire R1 are connected resistor 31 and condenser 38 in parallel.

Junction 30 is connected by lead 39 to cathode 40 of amplifier M. This amplifier may be a simple triode having cathode 40, control grid 43 and anode 44. Control grid 43 is connected through resistor 45 to ground. Condenser 46 is connected between the cathode and grid. Anode 44 is connected to junction 48, thence through resistor 49 and wire 50 to junction 5|. Junction SI has resistor 52 connected between it and junction 53, while resistor 54 is connected between junction 53 and junction 56. Junction 59 is connected by lead 51 to cathode 60 of full-wave rectifier BI. Rectifier 6i has anodes 63 and 64' respectively. Anode 53 is connected by lead 66 to one end terminal of transformer secondary 83 having grounded center tap 69. Anode 64 is connected by lead It! to the other terminal of secondary 68. Transformer secondary 68 is one winding of power supply transformer I2 having primary '13. Primary I3 is connected by lead 14 to one pole of plug I5 for use in a conventional 110 volt, 60 cycle circuit. Plug 15 has its other pole connected by lead '16 to terminal ll of power switch I8. Terminal 1! of switch I8 has movable contact 19 associated therewith, said contact being adapted to cooperate with fixed contact connected by lead 8I to primary I3 of the power transformer. is connected across primary 13.

Power transformer I2 is provided with low voltage winding 83 having grounded center tap 04 and supply wires 85 and 86. Connected across wires 85 and 8B is indicating light 81 to show when the master station is on. Wires 85 and 06 are connected to the heaters of the various amplifier tubes for energizing the same.

Grounded filter condensers 90, 9I and 92 are connected respectively to junctions 5I, 53 and 56. Referring to junction 48, the output of amplifier 4| is fed through blocking condenser 94 to junction 95, thence to control grid 96 of amplifier 91. Amplifier 91 has cathode 98 connected by lead 99 to grounded bias resistor I00. Junction 95 is also connected to grounded grid resistor IIII and grounded condenser I02. Amplifier 91 has accelerating grid I03, suppressor grid I64 and anode I05. Suppressor grid I04 is connected to lead 99 and to grounded by-pass condenser I08.

Accelerating grid I03 is connected to junction IIO. Between junction H0 and lead 50 is resistor III. Junction H0 is connected through blocking condenser-I I2 to junction I I3. Junction H3 is connected through resistor II4 to ground By-pass condenser 82 4 and is also connected through resistor IIB to junction II'I. Junction II! is also connected to ground through resistor I22.

Anode I95 of amplifier 91 is connected by lead I24 and load resistor I29 to wire 50. Wire I24 is connected through blocking condenser I2! to junction I28, which point is connected to ground through resistor I29. Junction I28 is connected by lead I30 to control grid I3I of amplifier I32. Amplifier I32 is preferably of the type generally designated as power amplifiers for audio frequency systems and is capable of handling moderate amounts of audio frequency power. Thus, this amplifier may be of the so-called beam power type of which the 6V6GT is an example. Amplifier I32 has cathode I33, accelerating grid I34, beam control electrodes I35 and anode 36. Accelerating grid I34 is connected by wire I31 going to junction 5|. Beam control electrodes I35 are maintained at any suitable potential such as by means of the cathode connection thereto. Cathode I33 is connected to ground by bias resistor I38 shunted by condenser I39. Anode I36 is connected to wire I48 which is connected by resistor I4! to lead I24. Wire I49 is connected to primary I43 of audio frequency output transformer I44. Primary I43 has its other terminal connected to junction 53. Audio frequency transformer I44 has secondary I45 whose one terminal I46 is grounded and whose other terminal I41 is connected to wire I48.

From junction 34 on wire 43, wire Si goes to junction I58, from which point a connection is made to switch contact I60 of the talk-listen switch. From junction I58, wire SI continues to contact S of distributing block I63. Distributing block I 63 has contacts A to S inclusive of which contacts 0, P and R are grounded. Contacts L and M are connected to complete the circuit between wires LI and MI.

It is understood that the number of useful contacts on the distributing block will vary depending upon the capacity of the master station. In the system herein described, a total of ten stations may be served.

Contact N is connected by lead I81 to terminal I88 of switch I8. Terminal I88 has movable contact I89 cooperating with fixed contacts I and I9I respectively. As is clearly indicated in the drawing, switch 78 has movable contacts I9 and I89 tied together. When the 110 volt, 60 cycle circuit is open, as indicated in the drawing, by contact I9 being away from contact 80, then movable contact I89 is disposed against contact I9I. When the 110 volt circuit is closed, movable contact 79 will touch fixed contact 89, while movable contact I09 will touch fixed contact I90. It is understood that movable contacts 19 and I89 are insulated from each other.

Contacts A to K inclusive are connected by wires AI to KI inclusive to station selector switch I95.

Station selector switch I has a series of fixed contacts 2A to 2K inclusive. The 2A to N series of contacts are normally connected together by wire I 96. Wire I96, at the bottom end, is connected by lead 200 to switch contact 20I of the talk-listen switch. Wire I96 at the top end, is connected by wire 202 to contact 203 of talklisten switch 23.

Station selector switch I95 has movable contacts 3A to 3K inclusive cooperating with the I and 2 series of contacts. Thus, each of the movable contacts may be moved to connect a pair of I and 2 contacts as desired. Station selector switch I95 also has a 4 series of fixed contacts 4A to 4K and a 5 series of contacts 5A to 5K respectively. Movable contacts 6A to 6K inclusive form a 6 series and are provided for cooperating between the 4 and 5 series of contacts.

The K portion of station selector switch I95 has additional fixed contacts IK and 8K respectively. 'IK is normally connected to IK by movable contact 3K in the position as shown. Similarly, 8K is connected to 4K normally by movable contact 6K.

Station selector switch I95 is so arranged that a pair of movable contacts in the 3 and 6 series for the same letters are operated together to open or close contacts simultaneously in the I, 2, 4 and 5 series. Thus, for example, movable contacts 3A and 6A are operated together so that contacts IA and 2A are closed and 4A and 5A are closed simultaneously. The same is true of the remaining contacts.

When movable contacts 3K and 6K are simultaneously moved, the connection between 1K and IK, on the one hand, is broken and the connection between 4K and 8K, on the other hand, is broken. With the movement of movable contacts 3K and 5K from the position shown, connections are established between IK and 2K, on the one hand, and between 4K and 5K, on the other hand.

The 4 series of contacts from A to J inclusive are connected together by wire 249 going to contact 24I of talk-listen switch 23. The 5 series contacts from A to J inclusive are connected by wire 243 having bottom end 244 grounded.

Talk-listen switch 23 has movable contacts 247 to 250 inclusive. These movable contacts are all mechanically tied together as indicated by the arrows and are adapted to be moved as a unit and assumetone of two positions. The position shown is a listening position. When moved to a talk position, all the movable contacts are moved downwardly as seen in the figure.

Contact I99 of power switch I8 is connected by wire 25I to fixed contact 4K of station selector switch I95. Fixed contact 8K is connected by wire 252 to junction In. Contact 5K is connected by wire 253 to junction II3. Contact I9I 01" power switch 18 is connected by wire 254 to junctions 255 and 256. Junction 255 is connected by wire 25! to contact 1K of the station selector switch. Contact IK is connected to wire I48 going to terminal I4! of output transformer secondary I45. Junction 256 is connected to contacts 269, 2IiI and 292 of talk-listen switch 23.

The master station disclosed herein is connected through a suitable cable going to the other stations in the system. Letters A to J inclusive indicate nine other stations in the system, while letter K represents a special station condition. It is, therefore, evident that a complete ten station system including the present station disclosed in detail is possible. Wires A2 to J2 inclusive will go from the corresponding contacts in dis tributing block I99. As is well known in this art. wire A2 for example will not necessarily be connected to the A contact of the distributing block of another master station.

In addition to wires A2 to J2 inclusive, wires N2, P2, R2 and S2 are provided. Wire N2 may be considered as a normal input from another master station. Wire R2 is the common return for remote stations. Wire P2 is the common return when this station is connected to some other master station. The other wire for the output would be any one or more of wires A2 to J2 inclusive.

It is understood that, with reference to the station disclosed herein, input conditions refer to circuit conditions when another station is calling this station, while output refers to circuit conditions when this station is calling another station. Wire S2 is provided in the event that a remote station is to originate a call to this station.

Station selector switch I is wired as shown in the drawing and is adapted for use in selecting one or more of nine stations in a ten-station system. As shown here, the wiring of selector switch I95 would call for all stations in the sys tern to be master stations. However, it is not necessary that any of the other stations in the system be of the master type. In order to provide for a mixed master and remote station system, wires I96 and 243 may be out along horizontally aligned points. The switches above the cut points will control master stations, and switches below the cut points will control remote stations. If three master stations, for example, are desired, wire 243 may be out between connections 5C and 5D. Similarly, wire I95 should be cut between 20 and 2D.

The operation of the station here may be analyzed for conditions when power switch I9 is off and on. If power switch I8 is off, the station may receive calls only from other master stations in the system butcannot have any output, 1. e. talk back. In that case, the output from another station will come through wire NIB into contact N of block I93. The circuit will continue through wire I81, switch point I88, switch blade I89, contact I9I along wire 254 to contact 260 of talk-listen switch 29. Assuming that the talk-listen switch is in the listening position as shown, the circuit will continue from contact 299 through movable contact 248 to fixed contact 2?, then along wire 25 to wiper 25. From wiper 25, the circuit will continue through resistor I5 to terminal I3, through cable MI to contact M, then to contact L and then to wire LI through speaker ID to junction I I. Junction I I is a ground for the system and is connected to contact R of block I63. From contact R, wire R2 is the common return.

Now assume that power switch I9 is in the on position. An incoming call from another master station may be traced from wire N2 through wire I81 to switch point I88, then to switch point I 90. From switch point I99, the circuit con tinuous through wire 25I, to contacts 4K, 6K and 8K through wire 252 to junction III. From junction III, a voltage dividing network is provided for a purpose which will be explained later. Useful energy will continue from junction II'I through resistor I I5 to junction I I3. From junction H3, audio frequency potentials go through blocking condenser I I2 to grid I93 of amplifier 91. The output of amplifier 9'! goes from anode I95 alon lead I24 through blocking condenser I2'I to grid I3I of amplifier I32. The output of amplifier I32 is fed through to primary I93 of transformer I44. Secondary I45 feeds potentials to wire I48 to contact IK. From contact IK, the circuit continues through contacts 3K, 'IK along wire 25? to junction 255 to junction 259 to contact 260 of talk-listen switch 23. From contact 269, the circuit continues through movable contact 248 to fixed contact 21 along wire 29 to wiper 25. From wiper 25, the circuit continues up resistor I5 to terminal I3, thence through wires MI and LI through speaker I 0 and junc- 7 tion II. From-junction I I, the circuit" goes to common return R2.

Still assuming that the power switch is on and that talk-listen switch 23 is now in the talk position, the circuit may now be traced as follows. Inasmuch as speaker I is" the generator for the system, we may begin with junction II and go through speaker II). The circuit continues through wires LI and MI to terminal I3, resistor I to wiper 25. From wiper 25, potentials are fed along wire 26 to fixed contact 21, thence through movable contact 243 to fixed contact I60. From contact I60, the circuit continues to junction I58, wire SI to junction 34, then wire 33 to the network of resistors and condensers. Thus, potentials will be fed to the cathode of amplifier M.

The output of amplifier 4I may be traced from anode 44 through blocking condenser 94 to control grid 98 of amplifier 97. The output of amplifier 9'! goes from anode I85 through blocking condenser I2! to grid I3I of amplifier I32. Amplifier I32 feeds its output to primary I43 of transformer I44. Secondary I45 of this transformer feeds potentials to wire I48. From wire I48, the circuit continues through contacts IK, 3K, 1K to wire 251, junctions 255 and 256 to contacts Zfii and 203 of talk-listen switch 23.

It is necessary that the station selector button corresponding to the calling station be depressed. If a master station originates the call, then the circuit goes from contact 293 along wire 282 to the particular switch closed, say C for example. The circuit continues through 2C and IC to wire CI, contact 0 on block I63 and then out along C2 to the calling station, through the calling station and back through output common wire P2 to ground.

If the calling station is a remote station, the output circuit goes from contact 28I to contact ZIlI, wire 290 up to the proper switch, say G for example, through contacts 2G and IG to wire GI and contact G on block I83 and out on G2. The return is through common wire R2.

It should be understood that the input from any other master station is always from wire N2 through the master station as previously traced and back to R2. On the other hand, if the input is from a remote station, then upon operation of the station selector switch, the input will be through some other wire depending upon the particular station, such as for example J 2 and back through R2. The input from a remote station originating a call may come in on line S2 providing that the station selector switch is in the 4 position shown in the drawing. It is evident that, in this condition, the master station cannot talk back to the calling remote station but must select that calling station on station selector switch I95. Upon the operation of the station selector switch, both the input and output from this master station is on wire J2, for example, if that is the station calling and back on R2,

Upon operation of the K portion of the selector switch, the following changes are made. Thus, output transformer secondary I45 is connected to wire KI, the circuit going from terminal I4'l through wire I48 to contact IK, then through contact 3K to contact 2K. From wire KI, the circuit goes out to wire K2, then back through a common return. Wire K2 and the common return may feed a speaker having an unusually loud level of operation, such as might be required for paging. This speaker may, of course, be located in any desired spot. At the same time, re-

8 sistor l l'5- isshurited out, the circuit running as follows: an input from N2 coming from another master station through wire I87 to contact I88, switch blade I89 to contact I80, wire 25I to contact 4K, contact 5K, contact 5K, wire 253 to junction H3.

Under normal conditions, another master providing input from line N2 is connected. to junction I IT. This makes resistor I22 a load for the master calling this station. Resistors I I6 and I M merely provide a voltage divider for reducing the level to the input of amplifier S1 to a desirable level. It should be noted that condenser I I2 not only functions as a screen by-pass but also as a coupling condenser for the input of amplifier 9? when another master station is talking.

When a remote station is talking to the present master station, the input is to amplifier 4 I. It is, thus, evident that three stages of amplification are provided when this station is talking to another station or is listening to a remote station. Only two stages of amplification are provided at this station when this station is listening to another master station.

When another master station is calling this station for paging, the operation of the K switches results in the entire voltage appearing at input line N2 applied to junction I I 3 rather than a portion thereof as would normally be the case. Thus, input energy to two stages of amplification is provided at a higher than normal level thus resulting in output energy at transformer secondary I at an abnormally high level. In order to talk back from this station, it will be necessary to move the K switch to the position shown in the drawing and select the station calling.

When another master station is calling this station with power switch 18 in the off position, it should be noted that transducer I6 is in series with that portion of resistor I5 between terminal I3 and wiper 25. The remaining portion of resistor I 5 to terminal I6 and resistor I 8 is open. The same situation obtains when this station has power switch IS on and is talking to another master station. It should be noted that ground 244 is cut oil from wire 243 for master service. On the other hand, when this station is talking to a remote station, a ground is provided for resistor I8. Thus, from wiper 25, two shunt paths to ground are provided.

As has been pointed out previously, resistor I5 normally has a value high in comparison to that of I8 or transducer 20. Thus, as an example, transducer 10 may have an impedance of about ohms and resistor I8 may have a resistance of about 50 ohms. Resistor I5 in that case may have a resistance of the order of about 400 ohms. Thus, when this station has its power switch oil and is listening to another master station or with power on is listening to a remote station or with power on when this station is talking to another master station, the position of wiper 25 for controlling volume is not critical. However, when this station is talking to a remote station, the setting of wiper 25 is critical and will have a substantial effect on the signal level. In that case. resistor I8 is grounded so that the volume control is a shunt control in a relatively high impedance circuit.

Condenser 38 is to by-pass any radio frequencies or supersonic frequencies which may be picked up. Condenser 32 may be used to control frequencies by proper choice of value and also blocks any slow changes in potential across resistor 31 from bias resistor 3|. Resistor 31 is merely a load to keep condenser 32 correctly polarized.

In the actual embodiment of the system, it has been found that grounds Mt and Il may have some slight differences of audio frequency poten tial therebetween. In order to prevent cross-talk, the common returns R2 and P2 are provided. It is understood, however, that the two returns and the various grounds may be consolidated into one over-all ground.

What is claimed is:

1. In a master station for an intercommunication system, said master station including an audio-frequency amplifier having input and output terminals, a transducer adapted to function as a microphone or speaker, talk-listen switching means having talk and listen positions, a resistor having a resistance high in comparison to the transducer impedance having one terminal permanently connected to a transducer terminal, a wiper cooperating with said resistor for potentiometer action, connections from said wiper and free terminal of the resistor to said talk-listen switching means, connections from the amplifier input and output to said talk-listen switching means and free transducer terminal, said switching means connecting said transducer output to the amplifier input when in talking position or said transducer input to the amplifier output when in listening position, station selecting switching means and connections to other stations for communicating with other stations, certain of said other stations being of the master type with an active amplifier and remaining other stations being of the remote type without amplifiers, and connections between said station selecting switching means and talk-listen switching means and the free terminal of said resistor for shunting said resistor across said transducer only when said station is talking to another remote station and for leaving said free resistor terminal open at all other times whereby said wiper position is not critical at such all other times.

2. In a master station for an intercommunication system, said master station including an audio-frequency amplifier having input and output terminals, a transducer adapted to function as a microphone or speaker, talk-listen switching means having talk and listen positions, a resistor having a resistance high in comparison to the transducer impedance having one terminal permanently connected to a transducer terminal, a wiper cooperating with said resistor for potentiometer action, connections from said wiper and free terminal of the resistor to said talk-listen switching means, connections from the amplifier input and output to said talk-listen switching means and free transducer terminal, said switching means connecting said transducer output to the amplifier input when in talking position or said transducer input to the amplifier output when in listening position, station selecting switching means and connections to other stations for communicating with other stations, said station selecting switching means including an individual station switch for each of the other stations, a common lead for one side of each station switch, said common lead having one end open and the other end connected to the free transducer terminal, said common lead being adapted to be severed at any desired point between station switches so that station switches connected to one end of the common lead go to master stations with an active amplifier while station switches connected to the other end of the common lead go to remote stations without amplifiers and connections between said talklisten switching means and the other side of said individual switches so that operation of switches corresponding to remote stations only shunt said resistor across said transducer only when said station is talking to another remote station and for leaving said free resistor terminal open at all other times whereby said wiper position is not critical at such all other times.

3. In a master station for an intercommunioation system, said master station including an audio-frequency amplifier having at least two stages and input and output terminals for said amplifier and an interstage input, a transducer adapted to function as a microphone or speaker, talk-listen switching means having talk and listen positions, connections from said transducer to said talk-listen switching means, connections from the amplifier input and output and from the interstage input to said talk-listen switching means, station selecting switching means and connections to other stations for communicating with other stations, certain of said other stations being of the master type with an active amplifier and remaining other stations being of the remote type without amplifiers, said station selecting switching means including an individual switc for each of the other stations, a common wire for one side of each of the station switches, said common wire having both ends connected to the talk-listen switching means, said common wire being adapted to be severed at any desired point between station switches, the point of severance separating master from remote stations, said talk-listen switching means in the listening position connecting the remote station side of the common wire to the amplifier input and connecting the amplifier output to the transducer, said talk-listen switching means leaving the master station side of the common wire open in listening position, said connections from other master stations including said talk-listen switching means and a connection to said interstage input when said talk listen switching means is in the listen position, said talk-listen switching means in talk position connecting the transducer output to the amplifier input and the amplifier output to said station selector switching means for distribution to other stations.

4. In a master station for an intercommunication system, said master station including an audio frequency amplifier having at least two stages and input and output terminals for said amplifier and an interstage input, a transducer adapted to function as a microphone or speaker, a resistor having a resistance high in comparison to the transducer impedance having one terminal permanently connected to a transducer terminal, a wiper cooperating with said resistor for potentiometer action, talk-listen switching means having talk and listen positions, connections from said wiper and free terminal of the resistor to said talk-listen switching means, connections from the amplifier input and output and from the interstage input to said talk-listen switching means and free transducer terminal, station selecting switching means and connections to other stations for communicating with other stations, certain of said other stations being of the master type with an active amplifier and remaining other stations being of the remote type without amplifiers, said station selecting switching means including a pair of switches for each of the other stations, a common Wire for one side 11 of one of each pair of switches, said common wire having both ends connected to the talk-listen switching means, said common wire being adapted to be severed at any desired point between station switches, the point of severance separating master from remote stations, said talk-listen switching means in the listening position connecting the remote station side of the common wire to the amplifier input and connecting the amplifier output to the transducer, said talklisten switching means leaving the master station side of the common wire open in listening position, said connections from other master stations including said talk-listen switching means and a connection to said interstage input when said talk-listen switching means is in the listen position, a common lead for one side of the other of each pair of station switches, said common lead having a master station end open and remote station end connected to the free transducer terminal, said common lead being adapted to be severed between switches corresponding to the same stations as in the common wire severance, and connections between said talk-listen switching means and the other side of the other of 12 each pair of station switches for shunting said resistor across said transducer only when said station is talking to another remote station and for leaving said free resistor terminal open at all other times whereby said wiper position is not critical at such all other times.

5. The system of claim 1 wherein said amplifier is of the vacuum tube type with said tube having cathode, grid and anode electrodes and wherein the amplifier input is to the cathode.

WILLIAM R. TORN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,054,933 Frazier Sept. 22, 1936 2,246,331 White et a1. June 17, 1941 2,258,807 Pitkin Oct. 14, 1941 2,302,798 Percival Nov. 24, 1942 2,339,678 Campbell Jan. 18, 19 .4 2,377,672 Campbell June 5, 1945 

